Covert action, the branch of intelligence work concerned with influencing events rather than simply gathering information, has rarely been a fashionable subject. It's a mark of the unusual moment that we are in that, thanks to allegations about Russian interference in the US, Rory Cormac's book appears somewhat timely.

Disrupt and Deny is billed as the first history of British covert action, and although Bloch and Fitzgerald's 1983 book might have a prior claim to that title, it certainly breaks new ground in its use of archival sources, underlining once again that British intelligence studies has moved some way beyond the official and semi-official histories of a decade ago.

Cormac takes 1945 as his starting point, a logical choice given the volume of writing on the wartime exploits of the Special Operations Executive, although it deprives us of what might have been a very timely discussion of British Security Co-ordination's political activities in the United States.

The failure of early attempts at SOE-type operations in post-war Albania led Britain to adopt a cautious 'pinprick' strategy during the early Cold War. Britain's imperial interests provided the context for more ambitious operations, notably 1953's Operation Boot, which overthrew Iranian Prime Minister Mohammad Mossadegh after he had nationalised the country's oil industry.

The fateful success in Iran provided a model for similar attempts in Syria and Egypt later in the decade. Cormac argues that the Suez crisis only increased British reliance on covert action to shore up declining conventional power.

The Middle East remained a focal point for British covert action, arranging the replacement of local rulers in the future United Arab Emirates and most spectacularly in Oman, where British military personnel were directly involved in the 1970 coup. Oman, along with Yemen, was also a key theatre of covert military intervention.

The role of mercenaries in these conflicts prompts Cormac to one of a number of tentative but useful forays into the relationship between official and private covert action. Others focus on the international intelligence forum known as 'Le Cercle' and the British counter-subversion group Shield, which lobbied Margaret Thatcher in the early 1980s.

Other 1960s conflicts examined include the Indonesian Confrontation, which illustrates the extent of continuity in covert action policy between Labour and Conservative governments, and the Congo, where the role of MI6's Daphne Park, later Baroness Park, provides a significant case study of the morality of covert action.

Park allegedly admitted over tea at the House of Lords that MI6 organised the murder of Congolese leader Patrice Lumumba. Cormac discounts this as a dry joke. Instead, he cites the conclusion of Park's biographer that neither MI6 nor the CIA 'pulled the trigger or gave the order to the firing squad, but they conspired to bring about a situation where the most logical, indeed the only possible outcome would be the death of Patrice Lumumba.'

This clearly fits with Park's recorded comment that 'they destroy each other, we don't destroy them', but is it really so far from her alleged statement that 'I organised it'?

What Cormac calls 'indirect complicity' might also be called collusion, a subject that comes up in a chapter on Northern Ireland in which he makes good use of previous work and valuable contributions of his own.

The latter includes important work on the Joint Action Committee (JAC), a little-known counterpart to the Cabinet Office's Joint Intelligence Committee, with which it shared a secretary, MI6 officer Brian Stewart. According to Cormac, Stewart pushed the full range of covert action in Northern Ireland in the early 1970s, from the staple tactic of deniable propaganda up to assassination. Cormac's evidence for this is largely based on private sources, with a sliver of archival support. This is perhaps inevitable, and marks out an area with potential for future research, however elusive the JAC files may prove to be.

Cormac makes a good job of disentangling the army's early covert units in Northern Ireland, the Military Reaction Forces, and their successors, although whether he is right to accept that '14th Intelligence' was ever more than a cover name in this context is open to question.

Cormac is also perhaps the first academic intelligence historian to make use of Anne Cadwallader's Lethal Allies. He notes a number of policies that created a climate for collusion, such as reliance on local forces known to be infiltrated by paramilitaries, the non-proscription of loyalist forces that were known to be active in terrorism, and the persistent refusal to provide the security forces with agent-handling guidelines.

He nevertheless argues that while the British government did authorise black propaganda and lethal covert operations, there was no policy of collusion.

One might however, argue that persistently turning a blind-eye to mid-level collusion was itself a form of high-level collusion, and indeed that it is just such a indirect, tacit, relationships that the very word collusion captures most precisely.

This was the view of the Canadian judge Peter Cory, who was appointed to examine the issue in the wake of the Good Friday Agreement. he wrote:

Because of the necessity for public confidence in government agencies the definition of collusion must be reasonably broad when it is applied to such agencies. That is to say that they must not act collusively by ignoring or turning a blind eye to the wrongful acts of their servants or agents or by supplying information to assist those servants or agents in their wrongful acts or by encouraging others to commit a wrongful act.

Any lesser definition would have the effect of condoning or even encouraging state involvement in crimes, thus shattering all public confidence in governmental agencies.

Subsequent inquiries, including the De Silva report (an important source for Cormac) have rowed back from Cory's definition. This is is all the more consequential because of their findings which clearly constitute collusion, if Cory's definition is applied.

Cormac's account is therefore a fair reflection of the state of a debate in which arguments about facts have begun to give way to arguments about definitions.

A previous reviewer, Robin Ramsay of Lobster, has said of this book: 'although the author has written an account which supports all the left critiques of imperialism and colonialism since WW2, he is not on the left,' and it seems clear that the implied audience is more policy-oriented than Bloch and Fitzgerald's post-Watergate journalism.

Candour as to the facts and reticence as to their normative implications is perhaps not an indefensible position for an academic historian. Nevertheless, there is much in Cormac's account of the British way in covert action to inform those wider debates, in Northern Ireland and elsewhere.